The measurement of oxidation-reduction potential, also called ORP or redox potential, is useful for characterizing the chemical properties of substances, usually aqueous solutions. The value of the redox potential is indicative of the presence and relative concentrations of oxidizing or reducing substances in a solution. For example when applied to the characterization of natural waters, the redox potential may indicate whether the normal level of dissolved oxygen has been depleted by chemical pollutants or biological activity. The measurement is made using a potentiometric cell consisting of an inert metal electrode, usually platinum, and a standard reference electrode. The standard reference electrode may be the normal hydrogen electrode (NHE), the saturated calomel electrode (SCE), or various other reference electrodes, for example, those based on silver in combination with silver chloride.
Since meaningful comparison of redox potentials can only be made when the type of cell used to make the measurement is specified, and since a user may not know the exact nature of the reference electrode when comparing cells from different manufacturers, calibration solutions are used to null out cell differences. These solutions contain xe2x80x9creversible redox couplesxe2x80x9d in the parlance of electrochemistry. A redox couple is a pair of species composed of the same element, but in different states of oxidation. These species are interconvertible by means of a redox reaction, that is, a chemical reaction which involves gain or loss of an electron or electrons. Redox couples which are readily interconvertible, that is, which show no resistance to undergoing interconversation when the redox potential of a solution changes due to addition of oxidizing or reducing substances, are referred to as reversible redox couples. It is these reversible redox couples which are useful in formulating redox standard solutions. Unfortunately, most redox couples are not stable in the presence of air and water; they usually revert entirely to one or the other of the two oxidation states. Thus the choice of redox couples suitable for formulating redox standard solutions is limited.
Two redox standard solutions are described in Standard Methods for the Examination of Water and Wastewater, 18th Ed., 1992. The first, called Zobell""s Solution, contains the redox couple iron(III)hexcyanoferrate and iron(II)hexacyanoferrate, also known as ferricyanide/ferrocyanide. The second standard solution, called Light""s Solution, contains the redox couple iron(III) and iron(II), also know as ferric/ferrous. Each of these redox standard solutions has its disadvantages, as discussed below.
Zobell""s Solution is known to be unstable when exposed to ordinary indoor lighting. In addition, this solution carries the stigma of containing cyanide groups. Light""s Solution is not light-sensitive and is generally more stable than Zobell""s Solution, but Light""s Solution is so strongly acidic (pH about 0.3) that it can easily burn the skin of the user and it readily destroys fabrics such as cotton.
In view of the disadvantages of these two commonly employed redox standard solutions, there is a clear need for a safe and reliable redox standard solution. The present invention fills this need.
As discussed in greater detail below, the present invention is based upon the use of a known redox couple, the triiodide/iodide redox couple. One previous use of an triiodide/iodide redox couple in a mixture with other ingredients is taught in U.S. Pat. No. 4,495,050. This patent, the disclosure of which is hereby incorporated herein by reference, describes and claims a potentiometric electrode which includes an internal, ionically-conductive filling or electrolyte which is electrically coupled to an external lead. As taught therein, the electrode includes a body of electrolytic material within a container, in which the electrolyte consists of a phosphate buffer, (0.05 M NaH2-H-PO4) to fix the activity of hydrogen ions, and a redox couple consisting of 4 molar potassium iodide (KI) and 0.0067 molar iodine (I2) in water (H2O), buffered with 0.02 molar boric acid (H3BO3), adjusted to a pH of 7.15 with potassium hydroxide (KOH). The reference half cell has the same lead and electrolyte, although the buffer may be omitted. Nothing in this patent either teaches or suggests the independent use of an aqueous solution consisting essentially of the triiodide/iodide redox couple for the formation of a redox standard calibration solution as taught and claimed herein.
One embodiment of the present invention is directed to a redox standard solution which is both safe (e.g., to skin and fabrics) and more stable (e.g., to heat and light) than either Zobell""s Solution or Light""s Solution. The redox standard solution of the present invention consists essentially of an aqueous solution containing the redox couple triiodide/iodide. Advantageously, the present invention consists essentially of an aqueous solution containing a maximum of about 0.01 moles per liter of iodine and a minimum of at least about 1 mole per liter of potassium iodide. As discussed in detail below, such a redox standard solution overcomes the disadvantages of the two prior art standard solutions, providing the user with a safe, stable and reliable standard solution for ORP measurement calibrations.
Another embodiment of the present invention is directed to a method of calibrating a potentiometric cell comprising an inert metal electrode and a standard reference electrode;
said calibration method comprising measuring the oxidation-reduction potential of a redox standard calibration solution which standard solution consists essentially of an aqueous solution containing the triiodide/iodide redox couple. Preferably, the calibration measurement is made using a standard solution which consists essentially of an aqueous solution containing up to about 0.01 moles per liter of iodine (I2) and at least about 1 mole per liter of potassium iodide. Most preferably, the calibration measurement is made using a standard solution which consists of an aqueous solution of 4 molar potassium iodide (KI) and 0.0067 molar iodine (I2).